In compliance with the will of the dead Khaan, a Grand Khurildai at Kharkhorum in 1229 selected Ogedei, third son of Chinggis, as Khaan. The Grand Khurildai also decided to launch a campaign against the Bulghars, Turks in the region of Kazan on the middle Volga River, and to complete the conquest of the outlying Western Xia territories. By 1229 Batu Khan, grandson of Chinggis, had defeated most of the Bulghar outposts, and in 1231 Ogedei sent an expedition to conquer the Korean Peninsula.
That same year, Ogedei decided to destroy Jin. He formed an alliance with the Song, then sent Tului southward with a large army into Jin territory. In 1232 in the middle of the campaign, Tului died, and Subeedei took command. He continued on to besiege Kaifeng, the Jin capital. Despite the defenders' skillful use of explosives, the city fell to the Mongols after a year's siege. Subeedei then completed the conquest of the Jin Empire, driving many of the Jurchen back into their original homeland, but absorbing others into the Mongol army for the further conquest of China.
Ogedei refused to divide the conquered region with the Song, which in 1234 attempted to seize part of the former Jin Empire. This was the signal for another war, which lasted forty five years.
Ogedei committed the Mongols, whose total population could not have exceeded 1 million, to an offensive war against the most populous nation on earth, while other Mongol armies were invading Iran, Anatolia, Syria, and the steppes of western Siberia and Russia. By this time, ethnic Mongols were a minority of the Mongol armies. The remainders were Turks, Tatars, Tanguud, Cumans, Bulghars, and other Inner Asian peoples. Nonetheless, the confidence with which the Mongol armies embarked on these farflung wars was almost as remarkable as the invariable success of their operations.
Ogedei's name is associated with a number of innovations in the development of the Mongol Empire. He set up a regular horse-relay post service (morin ortoo), which lasted until the mid-20th century, for fast communication throughout the empire and to ensure safe travel for dignitaries.
In compliance with the wishes of Chinggis, as expressed presumably in his legal code, the Ikh Zasag, his vast empire had been apportioned among his sons (only three survived; the eldest, Zuchi, had died in 1227), and his sons' descendants, subject to the overall authority of the Khaan at Kharakhorum, which was rebuilt in 1235 by Ogedei. Zuchi's son, Batu, ruled the region to the north and the west of Lake Balkash. Tsagaadai, the second son of Chinggis was given the southwestern region that includes modern Afghanistan, Turkestan and central Siberia. He and his successors were known as the khans of the Tsagaadai Mongols. By implication, this realm extended indefinitely to the southwest, as Batu's did to the northwest. Ogedei and his progeny were awarded China and the other lands of East Asia. Tului, the youngest of the four principal heirs, was to have central Mongolia, the homeland, in accordance with Mongol custom.
He and his descendants, however, were to share Mongolia's precious fighting manpower with the other three khanates.
The Khurildai of 1235 authorized at least two more major offensive operations: one against Tibet and the other in Eastern Europe. The Tibetan expedition was led by Godan, son of Ogedei, and the conquest was completed in 1239.
Source:http://www.mongoliatourism.gov.mn/